Surgical device actuated using asymmetric spring system

ABSTRACT

A compliant surgical device such as a flexible entry guide employs tendons to operate or steer the device and attaches asymmetric or constant force spring systems to control tension in the tendons. As a result, the surgical device can be compliant and respond to external forces during a surgical procedure without rapidly springing back or otherwise causing a reaction that damages tissue. The compliance also permits manual positioning or shaping of the device during or before insertion for a surgical procedure without damaging the tendons or connections of the tendons within the device or to a backend mechanism.

CROSS-REFERENCE TO RELATED APPLICATIONS

This patent document is a divisional of U.S. patent application Ser. No.12/494,797, filed Jun. 30, 2009, which is hereby incorporated byreference in its entirety.

BACKGROUND

Minimally invasive surgical techniques generally attempt to performsurgical procedures while minimizing damage to healthy tissue. Oneparticular technique for achieving this goal employs flexible surgicalinstruments that are able reach a target work site inside a patient byat least partially following a natural lumen such as the digestive tractof the patient. Following the natural lumen allows a surgeon to operateon the work site with less need for incisions made through healthytissue, although an incision may be needed at locations where theflexible instrument enters or leaves a natural lumen. An entry guide canbe used during such a surgical procedure to facilitate insertion andremoval of surgical instruments or tools during the procedure. Ingeneral, the entry guide is inserted through an incision or a naturalorifice and steered along a path to a point where the distal end of theentry guide nears or reaches a target work site. The entry guidegenerally contains one or more instrument lumens through which differentsurgical instruments can be inserted or removed. This allows instrumentsto be changed without requiring a delicate steering procedure each timea different set of instruments is needed.

Surgical instruments and entry guides that are able to follow a naturallumen or other convoluted paths generally must be flexible, whichrequires these devices to have properties and abilities that are notneeded in most other surgical instruments. In particular, although anentry guide must be flexible enough to navigate a convoluted path, theguide ideally should provide a stable base at the work site formanipulation of an instrument or instruments inserted through the guide.Additionally, the guide should not change shape or react to externalforces in a manner that could unintentionally damage adjacent tissue.Cables or tendons may extend through all or part of an entry guide foractuation of mechanical features of the entry guide or steering of theentry guide along its path. In some advanced surgical systems, thesecables are robotically operated using motors and computer aided control.(As used herein, the terms “robot” or “robotically” and the like includeteleoperation or telerobotic aspects.) The forces applied through thetendons can be significant, both to overcome friction and because thelengths of entry guides and instruments can create long moment arms. Aflexible surgical device needs to control these relatively large forcesso that reactions or movements along the length of the device do notdamage the adjacent tissue of the patient.

SUMMARY

In accordance with an aspect of the invention, a compliant surgicaldevice such as an articulated entry guide employs tendons to operate orsteer the device and attaches constant force spring systems to controltension in the tendons. As a result, the surgical device can becompliant and respond to external forces during a surgical procedurewithout rapidly springing back or otherwise causing a reaction thatdamages tissue. The compliance also permits manual positioning orshaping of the device during or before insertion for a surgicalprocedure without damaging the tendons or connections of the tendonswithin the device or causing damage to a backend mechanism.

One specific embodiment of the invention is a surgical device such as anentry guide. The device includes a shaft having a movable member, atendon attached to the member, a constant force spring system, and acontrol mechanism. The constant force spring system is attached to thetendon, and the control mechanism controls the magnitude that theconstant force spring system applies to the tendon. The tension in thetendon can thus be independent of external forces moving the tendon butcontrolled to articulate the member.

Another embodiment of the invention is also a surgical device. Thisembodiment includes a shaft having a movable member, a tendon attachedto the member, and an asymmetric spring system attached to the tendon.The asymmetric spring system is such that a force applied by theasymmetric spring system to the tendon has greater dependence on alocation of a proximal end of the spring system than on a location ofthe tendon. A control mechanism can be connected to the proximal end ofthe asymmetric spring system.

Yet another embodiment of the invention is a method for operating asurgical device. The method includes inserting an articulated shaft ofthe surgical device for a surgical procedure, and using asymmetric orconstant force spring systems to maintain balancing forces on members ofthe articulated shaft.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a flexible or articulated entry guide and backend mechanismin accordance with an embodiment of the invention.

FIG. 2A illustrates connections of two tendons to asymmetric springsystems for control of a movable link within a surgical device inaccordance with an embodiment of the invention.

FIGS. 2B and 2C illustrate asymmetric spring systems in accordance withembodiments of the invention respectively using a torsion spring and aconstant force spring to produce a tendon tension that remains constantwith movement of the tendon but is adjustable through controlmechanisms.

FIG. 3 illustrates connections of three tendons to asymmetric springsystems for control of a movable link within a surgical device inaccordance with an embodiment of the invention.

Use of the same reference symbols in different figures indicates similaror identical items.

DETAILED DESCRIPTION

Compliance in an articulated surgical device such as a flexible entryguide is generally desirable to permit manual shaping of the device. Inaccordance with an aspect of the invention, tendons that connect toportions (e.g., mechanical links or vertebrae) in the flexible device toa backend mechanism are connected to spring systems that can accommodatemanual manipulation of the flexible portion of the device withoutdamaging the backend mechanism or connections of the tendons. Inaccordance with a further aspect of the invention, the spring systemcoupled to the drive tendons can be asymmetric or even a constant forcespring, so that the spring system does not cause large reaction forcesand the device does not rapidly spring back in response to externalforces. The compliance of the surgical device and the lack of springback may help to avoid tissue damage which might otherwise be causedduring a surgical procedure when the flexible device could be subject tochanging external forces.

FIG. 1 illustrates a flexible entry guide 100 in accordance with anembodiment of the invention. Entry guide 100 includes a flexible maintube 110 and a backend mechanism 120 at the proximal end of main tube110. Main tube 110 is flexible in that main tube 110 can bend as neededto follow a convoluted path, but main tube 110 may include a series ofrigid links or mechanical members that can act as articulated vertebraeto change the shape of main tube 110. Some exemplary articulatedstructures suitable for main tube 110 are described in U.S. Pat App.Pub. No. US 2007/0135803 A1, entitled “Methods and Apparatus forPerforming Transluminal and Other Procedures” to Amir Belson; and U.S.Pat App. Pub. No. US 2004/0193009 A1, entitled “Endoscope having a GuideTube” of Ross et al., which are hereby incorporated by reference intheir entirety. Additionally, the articulated structure in an entryguide can employ some of the same architectures found in articulatedwrists and similar robotic mechanism such as described in U.S. Pat. No.6,817,974, entitled “Surgical Tool Having Positively PositionableTendon-Actuated Multi-Disk Wrist Joint” to Cooper et al.; U.S. Pat. App.Pub. No. US 2004/0138700 A1, entitled “Flexible Wrist For Surgical Tool”of Cooper et al.; and U.S. Pat. No. 6,699,235, entitled “Platform LinkWrist Mechanism” to Wallace et al., which are hereby incorporated byreference in their entirety. A compliant sheath made from a rubber orplastic such as neoprene, pellethane, FEP, PTFE, Nylon, or similarmaterial can cover the links and other internal structure of main tube110 to provide a sealed enclosure for the internal mechanisms of theentry guide and to facilitate insertion and removal of main tube 110during a surgical procedure. Main tube 110 would typically have adiameter between about 8 mm and about 25 mm, depending on the intendeduse of main tube 110 and the number of surgical instruments to besimultaneously guided. The overall length of main tube 110 can beselected according to the types of procedures being performed, but atypical length may be about 60 cm or more.

Main tube 110 also includes one or more instrument lumens 112. Eachinstrument lumen 112 can be a flexible tube made of rubber, neoprene,pellethane, FEP, PTFE, Nylon, or other flexible material. Eachinstrument lumen 112 runs most of the length of main tube 110 andgenerally passes through openings in or lies on surfaces of the links ormembers that are part of the mechanical system for controlling the shapeof main tube 110. Each instrument lumen 112 can act to guide and houseflexible surgical instruments that may be used during a surgicalprocedure. In particular, when needed, a flexible surgical instrument(not shown) can be inserted into an opening 112A at a proximal end ofinstrument lumen 112 and slid through the instrument lumen 112 so that atool at the distal tip of the flexible surgical instrument emerges froman opening 112B at a distal end of the instrument lumen 112. Instrumentlumens 112 would typically have diameters sized for standardizedsurgical instruments, e.g., 5 mm or 8 mm, so that an instrument lumen112 can handle many different types of instruments, for example, variousshapes and types of forceps, scissors, scalpels, and cauterizinginstruments. When an instrument in an instrument lumen 112 is notcurrently needed, the instrument can be removed from that instrumentlumen 112 and replaced by another flexible instrument without the needfor a complex and time consuming steering process. Sensors and camerasor other vision systems could similarly be inserted through instrumentlumens 112. Such easily replaceable instruments or other surgicalsystems may have their own backend mechanisms and/or interfaces that canbe operated independently of backend mechanism 120. Alternatively oradditionally, main tube 110 may include surgical instruments, sensors,vision systems, fluid channels, or other surgically useful systems (notshown) that are not intended to be removed during a surgical procedure,and such systems may be mechanically or electrically operated through aninterface provided by backend mechanism 120.

Tendons 130 connect portions (e.g., mechanical links or fixed surgicalsystems) of main tube 110 to backend mechanism 120 and are shown in acut-out portion of FIG. 1 . Tendons 130 can be, for example, stranded orwoven cables, monofilament lines, or tubes made of metal or a syntheticmaterial that provides sufficient strength and flexibility for operationof the systems connected to tendons 130. Backend mechanism 120 generallyoperates as a transmission that pulls on tendons 130 when powered by amotor pack (not shown). Backend mechanism 120 includes an interface towhich the motor pack can be mechanically coupled. In the illustratedembodiment, multiple toothed wheels 122 engage respective motors thatrotate toothed wheels to control tensions in respective tendons 130 asdescribed further below. For robotic operation, a control system (notshown) including a user interface operated by a surgeon and a computerexecuting software can control the motor pack. A sterile barrier may beprovided between backend mechanism 120 and the main tube 110, so thatthe motor pack and any other systems connected to backend mechanism 120are not contaminated during a surgical procedure.

FIG. 2A schematically illustrates a portion 200 of an entry guide usingan asymmetric spring systems 210 in a backend mechanism 220 to controlthe respective tensions in tendons 230A and 230B coupled to a mechanicallink 240. For ease of illustration, only two tendons 230A and 230B,generically referred to herein as tendons 230, are shown in FIG. 2A andthe illustrated tendons 230 are attached to the same link 240. An actualentry guide may contain on the order of ten to in excess of one hundredlinks 240, and each link 240 may have one or more tendons 230 thatterminate at that link 240. In general, the entry guide may beunder-constrained, i.e., some links 240 may not be directly attached toor constrained by tendons 230, but may be displaced by the stiffness ofa sheath or skin (not shown) around links 240 or by a stiffening rodextending through links 240. In an alternative embodiment, distal endsof tendons 230 may be attached to different portions of a flexiblesheath to provide a continuum mechanism, which does not require links240 or a hinged mechanism but is flexed by forces that tendons 230 applyto the sheath.

Tendons 230 may have proximal ends attached to respective asymmetricspring system 210 in backend mechanism 220 when compliance is desired inthe attached link or mechanism of the entry guide. The entry guide mayadditionally include systems where compliance is not desired, and drivesystems (not shown) in backend mechanism 220 may employ mechanisms,which are well known in the art, for non-compliant driving of tendonscoupled the systems for which compliance is not desired.

Each spring system 210 in FIG. 2A includes a mechanical drive system212, a spring 216, and a cam 218. Drive system 212 converts rotationalmotion of driver motors 250 into linear motion, and spring 216 connectsto drive system 212 so that the linear motion of drive system 212 movesa proximal end of the spring 216. (Note that this conversion to linearmotion is not a required element, the proximal end of each spring 216may alternatively be attached to a cable that is wound around a pulleyor capstan, which if necessary may be provided with a brake to preventunwanted motion when the pulley or capstan is decoupled from a drivemotor.) Cam 218 has a first guide surface on which a cable 217 attachedto the distal end of spring 216 attaches and rides and a second guidesurface on which a portion of tendon 230 attaches and rides. Thesesurfaces of cam 218 are generally at different distances from a rotationaxis of cam 218, so that the ratio of the tension in a tendon 230 to thespring force from spring 216 is equal to the ratio of the radialdistance to the point where cable 217 separates from cam 218 to theradial distance to the point where tendon 230 separates from cam 218.Each surface of cam 218 may be a spiral surface that extends formultiple revolutions in order to provide the desired range of movementof the tendon 230.

The guide surfaces of cam 218 are further shaped to reduce or eliminatethe dependency of the tension in attached tendon 230 on the position ofthe link 240 attached to that tendon 230, and to the shape of the pathof the tendon between the cam 218 and the link 240. In particular, ifcam 218 were replaced with a pulley having only circular guide surfaces,pulling tendon 230 would cause a proportional increase in the stretch ofspring 216, and assuming that spring 212 obeys Hooke's law, a linearincrease in the tension in the tendon 230. To reduce the dependence ofthe tension on external force applied to tendon 230 or link 240, one orboth of the surfaces of cam 218 is not circular, but provides a variablemoment arm upon which either the tension in tendon 230 or the force fromspring 230 acts as cam 218 rotates. For example, rotation of cam 218that tends to stretch spring 216 can either decrease the moment arm atwhich spring 216 acts on cam 218 or increase the moment arm on which thetension in tendon 230 acts. As is known for constant force springs, theshape of cam 218 can be selected so that the tension in tendon 230remains constant as movement of tendon 230 causes rotation of cam 218,while at the same time, the spring force from spring 216 increases inaccordance with Hooke's law. Spring system 210 can thus act as aconstant force spring or alternatively just reduce the rate at whichtension in tendon 230 changes as tendon 230 unwinds from cam 218.

Embodiments of cams and suitable systems for producing constant forcesprings using linear springs are described in more detail in U.S. Pat.App. Pub. No. US 2008/0277552 A1, entitled “Mechanical Arm Including aCounter-Balance” of Eugene F. Duvall and U.S. Pat. No. 7,428,855,entitled “Counter Balance System and Method with One or More MechanicalArms” of Eugene F. Duval, which are hereby incorporated by reference intheir entirety.

Each mechanical system 212 controls the position of the proximal end ofthe corresponding spring 216 and thereby influences the amount ofstretch in the corresponding spring 216 and the tension in the attachedtendon 230. In operation, if a mechanical system 212 in a spring system210 pulls on the attached spring 216, the spring 216 begins to stretch,and if the link 240 and tendon 230 attached to the spring system 210 areheld fixed, the force that spring 216 applies to cam 218 increases andtherefore the tension in the attached cable 230 increases. Accordingly,the tension in a tendon 230 depends linearly (in accordance with Hooke'slaw, the moment arms of cam 218, and the spring constant of spring 216)on movement of the proximal end of spring 216, but each spring system210 behaves asymmetrically, i.e., has a much weaker response orotherwise, acts with constant force, non-linear dependence, or smallereffective spring constant in response to external forces that movetendon 230.

Each drive system 212 as mentioned above converts rotational motion,which may be provided by a drive motor 250 mechanically coupled to thedrive system 212, into linear motion of the proximal end of spring 216.In an exemplary embodiment, drive system 212 is a ball screw, whichincludes a threaded shaft 214 that provides a spiral raceway for ballbearings held within a bore of a ball nut 213. Ball nut 213 mechanicallycouples to a corresponding motor 250, so that as motor 250 turns, shaft214 moves into or out of the bore of gear 213. A ball screw can provideminimal friction even when applying or withstanding significant force toor from spring 216. However, other mechanical systems couldalternatively be employed to stretch spring 216. For example, a simplethreaded device could operate in substantially the same manner as a ballscrew but with greater friction. Alternatively, the proximal end ofspring 216 could be attached to a cable that wraps around a capstan, sothat a motor that drives the capstan could move the proximal end ofspring 216. A system of gears and levers could also be used to convertrotational motion to linear motion, or instead of converting rotationalmotion, a linear drive system such as a solenoid could be used to movethe proximal end of spring 216. The examples provided here simplyillustrate a few of the mechanical systems suitable for drive system212, but clearly many other mechanical systems could be employed to movethe proximal end of spring 216.

An adjustable constant force spring or asymmetric spring system is notlimited to use of linear or coils springs but can be constructed usingother types of spring elements. FIG. 2B illustrates an example of aspring system 210B that uses a torsion spring 216B to produce a tensionin a tendon 230 that is nearly independent of movement of tendon 230 butis adjustable using a drive motor 250. In system 210B, torsion spring216B has a distal end attached to a cam 218 so that rotation of cam 218changes the torsion in torsion spring 216B. The torque caused by torsionspring 216B on cam 218 thus varies (e.g., linearly) with the angle ofrotation of cam 218. However, tendon 230 is wrapped on a surface of cam218 that is shaped to change the moment arm on which tendon 230 acts sothat a constant tension in tendon 230 causes a torque that changes inthe same manner as torque from torsion spring 216B. As a result springsystem 210B acts as a constant force spring. However, the spring forceand tension in tendon 230 can be controlled by using motor 250 to rotatethe proximal end of torsion spring 216B. In particular, motor 250winding torsion spring 216B tighter (or looser) increases (or decreases)the tension in tendon 230. Accordingly, each spring system 216 of FIG.2A can be replaced with a spring system 216B, provided that thedifference in the direction of the interface between motors 250 and thespring systems 216 and 216B is accommodated.

FIG. 2C shows another alternative asymmetric spring system 216C, whichemploys a constant force spring 216C. Constant-force spring 216C is arolled ribbon of spring material that is relaxed when the ribbon isfully rolled up. As the ribbon unrolls, the portion of the ribbon nearthe roll produces the spring force. This spring force remains nearlyconstant as the ribbon unrolls because the portion of the ribbon thatproduces the spring force, i.e., the portion near the roll, has nearlythe same shape as the spring unrolls. Tendon 230 when attached to aouter end of constant force spring 216C will experience a constant forcefrom spring 216C as tendon 230 moves. However, an interface (e.g., atoothed wheel) 252 can be attached to the inner end of constant forcespring 216C so that a motor (not shown) can engage interface 252 andchange the constant force of spring 216C and the tension in tendon 250.Accordingly, each spring system 216 of FIG. 2A can be replaced with aspring system 216C.

FIG. 2A illustrates a configuration in which two tendons 230A and 230Bare coupled to the same link 240. Link 240 can be mechanicallyconstrained so that link 240 can only rotate about a single axis.Tendons 230A and 230B can then attach on the opposite side of therotation axis, so that pulling on one tendon 230A or 230B causes onedirection of rotation and pulling on the other tendon 230B or 230A causerotation in the opposite direction. In this configuration, link 240 willbe at rest when the forces, including external forces, frictionalforces, and the tensions in tendons 230A and 230B, on link 240 are inequilibrium. A change in external forces applied to link 240, forexample, by movement of a patient during insertion of the entry guide ofFIG. 2A or after the entry guide has been inserted, can cause link 240to move. Further, this movement will cause little or no change in thetension in tendons 230A and 230B since the spring systems 210 arerelatively insensitive to movement of tendons 230A and 230B. The entryguide does not respond to the external forces with rapidly increasingresistance, and spring back, which might otherwise occur with a constantlength positioning system. (In contrast, most robotic mechanics andcontrols are set up to hold a constant position with variable force, nota constant force with variable position as in the entry guide of FIG.2A.) In the case where spring systems 210 act as constant force springs,the entry guide can be fully compliant without spring back even in thelimit where friction is negligible. More generally, spring back can beavoided when increases in the tension in tendons 230 induced by themovement of the entry guide have less effect than does friction.

Link 240 in the entry guide of FIG. 2A can be moved by activating amotor 250 to turn a drive system 212 and change the tension in at leastone of tendons 230A and 230B. The change in tension unbalances theequilibrium of forces causing link 240 to move until a new equilibriumis established. In general, this may involve operating one mechanicalsystem 212 to stretch a corresponding spring 216 and increase tension inone tendon 230A or 230B. Optionally, the other mechanical system 210 maybe operated to relax tension in the other tendon 230B or 230A. When link240 rotates by the desired amount, tensions in the two tendons 230A and230B can be adjusted as required to re-establish equilibrium (e.g., backto their original tension settings.) In general, the positions of links240 do not have a fixed relation to the setting of mechanical systems212. However, the position of each link 240 (or the shape of the entryguide as a whole) can be visually observed by an operator or sensed, forexample, using a shape sensor such as described in U.S. Pat. App. Pub.No. US 2007/0156019 A1 (filed Jul. 20, 2006), entitled “Robotic SurgerySystem Including Position Sensors Using Fiber Bragg Gratings” by Larkinet al., and U.S. patent application Ser. No. 12/164,829 (filed Jun. 30,2008) entitled “Fiber optic shape sensor” by Giuseppe M. Prisco, both ofwhich are incorporated herein by reference. Movement of an entry guideemploying the system of FIG. 2A may thus be robotically controlled orcomputer assisted using a control system 260 and a sensor 270implementing a feedback loop that monitors the links 240 in the entryguide and controls drive motors 250, for example, to steer the entryguide during an insertion process. Steering an entry guide to follow anatural lumen generally does not require rapid or rigid response, sothat slow movement and use of forces just above the external resistanceand internal frictional force may be desired to minimize movement thatovershoots target position.

FIG. 2A illustrates one specific configuration of backend mechanism 220and spring systems 210 relative to a main tube of an entry guide.However, many other configurations can alternatively be employed. Inparticular, in FIG. 2A, the axis or rotation of gears 213 aresubstantially parallel to the direction from which the main tube of theentry guide extends from backend mechanism 220. If the spring systems210B or 210C of FIG. 2B or 2C were used, the rotation axis of controlmotors 250 would be perpendicular to the direction of entry guide. FIG.3 illustrates an alternative configuration using spring systems 210 buthaving a backend mechanism 320 using drive motors 250 with the rotationaxes that are substantially perpendicular to the main tube. Again,spring systems 210B or 20C of FIG. 2B or 2C can be used in place ofspring system 210 in the system of FIG. 3 .

FIG. 3 also illustrates a configuration in which three tendons 330A,330B, and 330C have distal ends attached to the same link 340. In thisconfiguration, link 340 may have a pivot system that allows rotation oflink 340 about two independent axes. The tensions in one or more oftendons 330A, 330B, and 330C can then be increased to tilt link 340 andthe tensions can be brought back into balance (with each other, externalforces, and friction) when link 340 reaches the desired orientation. Thethree tendons 330A, 330B, and 330C can thus be used to control twodegrees of freedom of link 240.

Although the invention has been described with reference to particularembodiments, the description is only an example of the invention'sapplication and should not be taken as a limitation. For example,although the above embodiments disclose specific embodiments of theinvention that are entry guides, embodiments of the invention may alsobe suitable for use in other surgical instruments where compliance isdesirable. Various other adaptations and combinations of features of theembodiments disclosed are within the scope of the invention as definedby the following claims.

What is claimed is:
 1. A surgical device comprising: a shaft including amovable portion; a first tendon comprising a first end and a second endand extending from the first end of the first tendon to the second endof the first tendon, the first tendon extending through the shaft, andthe second end of the first tendon being attached to the movableportion; a first asymmetric spring system comprising a first end and asecond end, the first asymmetric spring system extending from the firstend of the first asymmetric spring system to the second end of the firstasymmetric spring system, wherein the first end of the first asymmetricspring system is attached to the first end of the first tendon, andwherein the first asymmetric spring system is configured such that aforce applied by the first asymmetric spring system to the first tendonhas greater dependence on a location of the second end of the firstasymmetric spring system than on a location of the first end of thefirst asymmetric spring system; and a first mechanism connected tocontrol the location of the second end of the first asymmetric springsystem.
 2. The device of claim 1, wherein the force applied by the firstasymmetric spring system to the first tendon is independent of thelocation of the first end of the first asymmetric spring system and alocation of the first tendon while the location of the second end of thefirst asymmetric spring system remains fixed.
 3. The device of claim 1,wherein the shaft contains a lumen sized to guide a surgical instrumentthrough the shaft.
 4. The device of claim 1, wherein the shaft includesa plurality of members, each of which is articulated for control of ashape of the shaft.
 5. The device of claim 1, further comprising: asecond tendon extending through the shaft and attaching to the movableportion of the shaft; a second asymmetric spring system having a firstend attached to the second tendon, wherein the second asymmetric springsystem is such that a force applied by the second asymmetric springsystem to the second tendon has greater dependence on a location of asecond end of the second asymmetric spring system than on a location ofthe first end of the second asymmetric spring system; and a secondmechanism connected to control the location of the second end of thesecond asymmetric spring system.
 6. The device of claim 5, furthercomprising: a third tendon extending through the shaft and attaching tothe movable portion of the shaft; a third asymmetric spring systemhaving a first end attached to the third tendon, wherein the thirdasymmetric spring system is such that a force applied by the thirdasymmetric spring system to the third tendon has greater dependence on alocation of a second end of the third asymmetric spring system than on alocation of the first end of the third asymmetric spring system; and athird control mechanism connected to control the location of the secondend of the third asymmetric spring system.
 7. The device of claim 1,wherein the first asymmetric spring system comprises a spring elementand a cam attached between the spring element and the first tendon. 8.The device of claim 7, wherein the first mechanism comprises a ballscrew attached to the second end of the first asymmetric spring system.9. The device of claim 1, further comprising: a sensor in the shaft; anda control system that operates the first mechanism to actuate themovable portion of the shaft until the sensor indicates the shaft hasreached a desired configuration.
 10. A surgical device comprising: ashaft including a movable portion; a first tendon comprising a first endand a second end and extending from the first end of the first tendon tothe second end of the first tendon, the second end of the first tendonbeing attached to the movable portion of the shaft; a first asymmetricspring system comprising a first end and a second end, the firstasymmetric spring system extending from the first end of the firstasymmetric spring system to the second end of the first asymmetricspring system, wherein the first end of the first asymmetric springsystem is attached to the first end of the first tendon and isconfigured to apply a first force through the first tendon to themovable portion of the shaft, the first force having greater dependenceon a location of the second end of the first asymmetric spring systemthan on a location of the first end of the first asymmetric springsystem; and a first mechanism connected to move the second end of thefirst asymmetric spring system so that a magnitude of the first forcechanges and the movable portion of the shaft moves.
 11. The device ofclaim 10, wherein the shaft contains a lumen sized to guide a surgicalinstrument through the shaft.
 12. The device of claim 11, wherein aportion of the lumen passes through the movable portion.
 13. The deviceof claim 10, wherein the movable portion of the shaft includes aplurality of members, each of which is articulated for control of ashape of the shaft.
 14. The device of claim 10, further comprising: asecond tendon attached to the movable portion of the shaft; a secondasymmetric spring system having a first end attached to the secondtendon and applying a second force through the second tendon to themovable portion of the shaft, the second force having greater dependenceon a location of a second end of the second asymmetric spring systemthan on a location of the first end of the second asymmetric springsystem; and a second mechanism connected to move the second end of thesecond asymmetric spring system so that a magnitude of the second forcechanges and the movable portion of the shaft moves.
 15. The device ofclaim 14, further comprising: a third tendon attached to the movableportion of the shaft; a third asymmetric spring system having a firstend attached to the third tendon and applying a third force through thethird tendon to the movable portion of the shaft, the third force havinggreater dependence on a location of a second end of the third asymmetricspring system than on a location of the first end of the thirdasymmetric spring system; and a third mechanism connected to move thesecond end of the third asymmetric spring system so that a magnitude ofthe third force changes and the movable portion of the shaft moves. 16.The device of claim 10, wherein the first asymmetric spring systemcomprises a cam and a spring element having a first end coupled to thecam, the cam being coupled to the first end of the spring element toapply the first force to the first tendon and being shaped such that achange in the magnitude of the first force as the movable portion of theshaft moves through a range of motion is less than a change in amagnitude of a force that the spring element applies to the cam.
 17. Thedevice of claim 16, wherein the first mechanism comprises a ball screwattached to a second end of the spring element, and the first mechanismalters the first asymmetric spring system by moving the second end ofthe first asymmetric spring system relative to the cam.
 18. The deviceof claim 16, wherein the spring element comprises a linear springattached to a cable that wraps around a portion of the cam.
 19. Thedevice of claim 16, wherein the spring element comprises a torsionspring attached to the cam.
 20. The device of claim 10, furthercomprising: a sensor in the shaft; and a control system that operatesthe first mechanism to move the shaft until the sensor indicates theshaft has reached a desired configuration.